No backup, windows.
It really depends on how these files disappeared, but you could attempt to use undelete utilities to recover the files... there used to be a few free ones out there, I haven't used one in a while.
Have you looked directly at the location that the client keeps the wallet.dat?
if Vista/7/8, I believe this is C:\Users\YourUserName\Appdata\Roaming\BitCoin
Curious what files/folders are in there right now...
I've never had the client touch the wallet.dat file before, amongst many different corruptions of the blockchain and full re-downloads, so I would be surprised if it was the Bitcoin client itself that removed the wallet.dat... what version are you using?